The present preferred embodiment relates to the field of locking containers on load carrying platforms, such as on a railcar.
Association of American Railroads Specification M-952 spells out requirements for intermodal container securement devices with automatic latching and unlatching. Chief among these requirements are: 1) the downward force required to allow the container corner casting to engage the container lock; and 2) the upward force which must be attained before the container corner casting is allowed to disengage the container lock. Specifically, the engagement or latching force is 800 lbs. maximum and the disengagement or unlatching force must be between 1,600 and 2,200 lbs. Another requirement is that the physical engagement of the latching device into a locking aperture of the container corner fitting be not less than ¾″ under the most adverse conditions.
Latch type container locks as described in U.S. Pat. No. 5,570,981, and others, incorporate a single latch. The vertical post of the container lock is designed to allow a ¼″ lateral clearance between the post and a back corner edge of the locking aperture in the corner casting. Therefore, to maintain the ¾″ minimum lateral physical engagement that the latch must make with the locking aperture of the corner casting, the latch must laterally protrude at least ¾″ plus ¼″ or a total of 1″. Described in another way, the 1600-2200 lbs. disengagement force (600 lb range must occur when displacing the latch at least ¾″ laterally, or under worst conditions, 1″. As an example, the 1″ lateral travel could occur at 2200 lbs of force, equating to a spring rate of 2200 lbs/in. That same spring at ¾″ travel requires a force of ¾ times 2200 or 1650 lbs. Typical spring rate tolerances are +/−10% or +/−220 lbs (440 lbs total which is 73% of the 600 lb range). The tolerance acceptable on this spring would be a total of 50 lbs, or 25 divided by 2200 equals +/−1.1%. Not only does the force have to be repeatable but the design must also include any force due to sliding friction Therefore previous working prior art designs generally incorporated two separate springs, one each for the insertion and extraction operations respectively.